Problem: The scale on a map is 2cm : 9km. If two cities are 18cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 2cm on the map corresponds to an actual distance of 9km. A distance of 18cm on the map is the same as 9 $\cdot$ 2cm. The actual distance between the two cities is 9 $\cdot$ 9km, or 81km.